diff --git a/ch04_keys.adoc b/ch04_keys.adoc
index cc586066..35f1c9ec 100644
--- a/ch04_keys.adoc
+++ b/ch04_keys.adoc
@@ -70,7 +70,7 @@ generate a public((("public key cryptography", startref="pub-key")))((("key pair
 
 .Why Use Asymmetric Cryptography (Public/Private Keys)?
 ****
-Why is asymmetric
+Why is ((("asymmetric cryptography", see="public key cryptography")))((("public key cryptography", "purpose in Bitcoin")))((("digital signatures")))asymmetric
 cryptography used in Bitcoin? It's not used to "encrypt" (make secret)
 the transactions. Rather, a useful property of asymmetric cryptography
 is the ability to generate _digital signatures_. A private key can be